The present invention relates to hydraulic tensioners used with chain or belt drive systems, such as timing systems for internal combustion engines, and, more particularly, to hydraulic tensioners with a pre-loaded biasing effect to reduce peak operating loads of the system as they approach and achieve the maximum limit of a prescribed range for the tensioner and a pressure relief valve to provide relief from peak operating loads of the system that exceed the maximum limit of the prescribed range for the tensioner.
Hydraulic tensioners are typically used as a tension control device for chain or belt drive system, such as a timing system for an internal combustion engine. The tension in the chain or belt may vary greatly due to the wide variation in the temperature and the thermal expansion among the various parts of the drive system and any system along with which it operates. Thus, tensioners are used to impart and maintain a certain degree of tension on the chain or belt to prevent slippage from elongation and noise.
More specifically, for example, the timing chain system for internal combustion engines typically includes a chain that wraps about two spaced sprockets, which are commonly referred to as a crankshaft sprocket and a camshaft sprocket. In addition to temperature and thermal expansion effects caused by the engine, camshaft and crankshaft induced torsional vibrations can cause the chain tension to vary considerably. Reduction in chain tension also results from chain elongation and wear to the parts through prolonged use. Chain elongation can cause undesirable noise and slippage, which can possibly cause serious damage to the engine and other components by altering the camshaft timing by several degrees. Thus, it is important to impart and maintain appropriate tension on the chain to take up slack due to elongation to prevent slippage of the chain about the sprockets and noise.
Hydraulic tensioners typically include a housing having a bore, a fluid chamber defined by the bore and a hollow piston biased outward of the bore by a spring. A check valve is provided to permit fluid flow from a source of pressurized fluid into the fluid chamber, while preventing back flow in the reverse direction. The spring and the hydraulic pressure in the housing force the piston outward to impart and apply tension against the chain or belt to take up the appropriate amount of slack.
When the piston tends to move in the reverse direction, the check valve closes to restrict outflow of the fluid from the chamber. Although a small clearance between the piston and the housing wall may permit small amounts of the fluid to escape, the tensioner achieves a so-called xe2x80x9cno-return functionxe2x80x9d because the piston is virtually unable to retract. The no-return function presents potential problems for the hydraulic tensioner when tension spikes or surges in the chain or belt are encountered during operation. For example, when the timing system operates at its resonant frequency, the chain load increases significantly, and in many cases approaches (and in some cases can even exceed), the maximum limit of the prescribed range of operation for the tensioner. The small clearance between the piston and the housing wall is insufficient to quickly release the hydraulic fluid from the chamber to accommodate the sudden overload on the tensioner by the chain or belt.
An example of a tensioner directed to addressing operating loads at the maximum limit of the prescribed range is disclosed in Wigsten et al., U.S. Pat. No. 5,993,342, which is commonly owned by the assignee of this application. Wigsten et al. discloses a hydraulic tensioner with a pre-loaded limiting feature, such as a spring, at the upper end of the piston to reduce the peak operating loads in a chain system. The spring member is located between the primary piston and an upper secondary piston. This design works well to reduce the peak operating loads in the system up to the maximum limit of a prescribed range for a tensioner. As mentioned, there are instances, however, where the chain system produces peak operating loads beyond the realistic prescribed range to be accommodated by the tensioner. Thus, there is a need for the tensioner to be able to address these excessive loads and provide temporary relief.
An example of a tensioner directed to addressing operating loads at the maximum limit of the prescribed range is disclosed in Suzuki, U.S. Pat. No. 4,881,927. Suzuki discloses a hydraulic ball-type check valve tensioner having a piston slidably fitted into a primary chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve having a sleeve slidably fitted in an auxiliary chamber in communication with the primary chamber, with a spring biasing the sleeve into a depressed position to block a discharge port. Hydraulic fluid in the primary chamber flows into the auxiliary chamber to force the sleeve against the biasing spring action to unblock the discharge port. A shortcoming with this design is the potential for the relief valve to open and close slowly due to the relatively high mass of the components and the variable friction between the sleeve and auxiliary chamber wall. This shortcoming also may cause the pressure at which the relief valve operates to vary and otherwise be inconsistent.
Accordingly, there is a need for an improved tensioner that not only addresses operating loads as they approach and achieve the maximum limit of the prescribed range to provide a constant tensioning force, but that also addresses operating loads that exceed the maximum limit of the prescribed range in an effective and efficient manner to provide temporary relief from the no-return function to prevent damage to the tensioner and the drive system.
The present invention relates to hydraulic tensioners used with chain or belt drive systems, such as timing systems for internal combustion engines, and includes a pressure relief valve to provide relief from peak operating loads of the system that exceed the maximum limit of the prescribed range for the tensioner. The hydraulic tensioner includes a housing defining a bore and a primary piston slidably received within the bore. The primary piston defines a fluid chamber with the bore and has a lower end defining a first opening and an upper end defining a second opening.
The hydraulic tensioner further includes a secondary piston that extends through the second opening of the primary piston. The secondary piston includes a base portion and an upper end portion. The base portion is disposed in the primary piston and the upper end portion is spaced from the base portion and the primary piston. The secondary piston is permitted to move axially with respect to the primary piston.
A first biasing member is located within the bore to bias the primary piston outwardly of the bore. A second biasing member is located between the primary piston and the upper end portion of the secondary piston to bias the secondary piston away from the primary piston.
A first valve is provided between the fluid chamber and a source of pressurized fluid to permit fluid flow into the chamber while blocking flow in the reverse direction. A passage in the housing connects the fluid chamber with the source of pressurized fluid. A second valve discharges fluid flow from the chamber through the second opening of the primary piston to reduce hydraulic pressure in the chamber when the secondary piston moves axially towards the primary piston a predetermined stroke amount due to operating loads acting on the tensioner exceeding a maximum limit of a prescribed range for the tensioner.
The second biasing member also may be compressed a predetermined amount when interposed between the primary piston and the upper end portion of the secondary piston. The second biasing member may be further compressed when the operating load on the secondary piston exceeds a predetermined amount within the prescribed range of operation.
The second valve may further include a recess defined by a portion of the secondary piston between the base portion and the upper end portion. Thus, the second valve would be in an open position when the recess is located across the second opening of the primary piston when the operating load on the secondary piston moves the piston axially toward the primary piston the predetermined stroke amount due to operating loads exceeding the maximum limit of the prescribed range of operation so to discharge fluid from the chamber to reduce hydraulic pressure in the chamber. Since the secondary piston is capable of moving axially, the second valve is closed when the recess moves so that it is not across the second opening of the primary piston under the pressure of fluid flow into the chamber and the bias of the first and second biasing members when the operating loads reduce below the maximum limit of the prescribed range for the tensioner.
The hydraulic tensioner may further include a nose piece attached to the upper end portion of the secondary piston with the second spring being interposed between the primary piston and the nose piece. Further, the upper end portion of the secondary piston may have a threaded portion and the nose piece may define a bore with internal threading that complements the threaded upper end portion for attaching the nose piece to the upper end portion of the secondary piston.
The first biasing member may be a coil spring interposed between the upper end of the primary piston and the housing to bias the primary piston outward of the housing. The second biasing member may be a coil spring, a plurality of Belleville washers, a resilient material or compressed air interposed between the upper end portion of the secondary piston and the upper end of the primary piston to bias the secondary piston away from the primary piston.